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<pre><code>public static void main (String[] args) { //In order to understand this algorithm, it is necessary to understand the following: //int checker = 0; //Here we are using the primitive int almost like an array of size 32 where the only values can be 1 or 0 //Since in Java, we have 4 bytes per int, 8 bits per byte, we have a total of 4x8=32 bits to work with //int val = str.charAt(i) - 'a'; //In order to understand what is going on here, we must realize that all characters have a numeric value for (int i = 0; i < 256; i++) { char val = (char)i; System.out.print(val); } //The output is something like: // !"#$%&'()*+,-./0123456789:;<=>?@ABCDEFGHIJKLMNOPQRSTUVWXYZ[\]^_`abcdefghijklmnopqrstuvwxyz{|}~ ¡¢£¤¥¦§¨©ª«¬®¯°±²³´µ¶·¸¹º»¼½¾¿ÀÁÂÃÄÅÆÇÈÉÊËÌÍÎÏÐÑÒÓÔÕÖ×ØÙÚÛÜÝÞßàáâãäåæçèéêëìíîïðñòóôõö÷øùúûüýþÿ //There seems to be ~15 leading spaces that do not copy paste well, so I had to use real spaces instead //To only print the characters from 'a' on forward: System.out.println(); System.out.println(); for (int i=0; i < 256; i++) { char val = (char)i; //char val2 = val + 'a'; //incompatible types. required: char found: int int val2 = val + 'a'; //shift to the 'a', we must use an int here otherwise the compiler will complain char val3 = (char)val2; //convert back to char. there should be a more elegant way of doing this. System.out.print(val3); } //Notice how the following does not work: System.out.println(); System.out.println(); for (int i=0; i < 256; i++) { char val = (char)i; int val2 = val - 'a'; char val3 = (char)val2; System.out.print(val3); } //I'm not sure why this spills out into 2 lines: //EDIT I cant seem to copy this into stackoverflow! System.out.println(); System.out.println(); //So back to our original algorithm: //int val = str.charAt(i) - 'a'; //We convert the i'th character of the String to a character, and shift it to the right, since adding shifts to the right and subtracting shifts to the left it seems //if ((checker & (1 << val)) > 0) return false; //This line is quite a mouthful, lets break it down: System.out.println(0<<0); //00000000000000000000000000000000 System.out.println(0<<1); //00000000000000000000000000000000 System.out.println(0<<2); //00000000000000000000000000000000 System.out.println(0<<3); //00000000000000000000000000000000 System.out.println(1<<0); //00000000000000000000000000000001 System.out.println(1<<1); //00000000000000000000000000000010 == 2 System.out.println(1<<2); //00000000000000000000000000000100 == 4 System.out.println(1<<3); //00000000000000000000000000001000 == 8 System.out.println(2<<0); //00000000000000000000000000000010 == 2 System.out.println(2<<1); //00000000000000000000000000000100 == 4 System.out.println(2<<2); // == 8 System.out.println(2<<3); // == 16 System.out.println("3<<0 == "+(3<<0)); // != 4 why 3??? System.out.println(3<<1); //00000000000000000000000000000011 == 3 //shift left by 1 //00000000000000000000000000000110 == 6 System.out.println(3<<2); //00000000000000000000000000000011 == 3 //shift left by 2 //00000000000000000000000000001100 == 12 System.out.println(3<<3); // 24 //It seems that the - 'a' is not necessary //Back to if ((checker & (1 << val)) > 0) return false; //(1 << val means we simply shift 1 by the numeric representation of the current character //the bitwise & works as such: System.out.println(); System.out.println(); System.out.println(0&0); //0 System.out.println(0&1); //0 System.out.println(0&2); //0 System.out.println(); System.out.println(); System.out.println(1&0); //0 System.out.println(1&1); //1 System.out.println(1&2); //0 System.out.println(1&3); //1 System.out.println(); System.out.println(); System.out.println(2&0); //0 System.out.println(2&1); //0 0010 & 0001 == 0000 = 0 System.out.println(2&2); //2 0010 & 0010 == 2 System.out.println(2&3); //2 0010 & 0011 = 0010 == 2 System.out.println(); System.out.println(); System.out.println(3&0); //0 0011 & 0000 == 0 System.out.println(3&1); //1 0011 & 0001 == 0001 == 1 System.out.println(3&2); //2 0011 & 0010 == 0010 == 2, 0&1 = 0 1&1 = 1 System.out.println(3&3); //3 why?? 3 == 0011 & 0011 == 3??? System.out.println(9&11); // should be... 1001 & 1011 == 1001 == 8+1 == 9?? yay! //so when we do (1 << val), we take 0001 and shift it by say, 97 for 'a', since any 'a' is also 97 //why is it that the result of bitwise & is > 0 means its a dupe? //lets see.. //0011 & 0011 is 0011 means its a dupe //0000 & 0011 is 0000 means no dupe //0010 & 0001 is 0011 means its no dupe //hmm //only when it is all 0000 means its no dupe //so moving on: //checker |= (1 << val) //the |= needs exploring: int x = 0; int y = 1; int z = 2; int a = 3; int b = 4; System.out.println("x|=1 "+(x|=1)); //1 System.out.println(x|=1); //1 System.out.println(x|=1); //1 System.out.println(x|=1); //1 System.out.println(x|=1); //1 System.out.println(y|=1); // 0001 |= 0001 == ?? 1???? System.out.println(y|=2); // ??? == 3 why??? 0001 |= 0010 == 3... hmm System.out.println(y); //should be 3?? System.out.println(y|=1); //already 3 so... 0011 |= 0001... maybe 0011 again? 3? System.out.println(y|=2); //0011 |= 0010..... hmm maybe.. 0011??? still 3? yup! System.out.println(y|=3); //0011 |= 0011, still 3 System.out.println(y|=4); //0011 |= 0100.. should be... 0111? so... 11? no its 7 System.out.println(y|=5); //so we're at 7 which is 0111, 0111 |= 0101 means 0111 still 7 System.out.println(b|=9); //so 0100 |= 1001 is... seems like xor?? or just or i think, just or... so its 1101 so its 13? YAY! //so the |= is just a bitwise OR! } public static boolean isUniqueChars(String str) { int checker = 0; for (int i = 0; i < str.length(); ++i) { int val = str.charAt(i) - 'a'; //the - 'a' is just smoke and mirrors! not necessary! if ((checker & (1 << val)) > 0) return false; checker |= (1 << val); } return true; } public static boolean is_unique(String input) { int using_int_as_32_flags = 0; for (int i=0; i < input.length(); i++) { int numeric_representation_of_char_at_i = input.charAt(i); int using_0001_and_shifting_it_by_the_numeric_representation = 1 << numeric_representation_of_char_at_i; //here we shift the bitwise representation of 1 by the numeric val of the character int result_of_bitwise_and = using_int_as_32_flags & using_0001_and_shifting_it_by_the_numeric_representation; boolean already_bit_flagged = result_of_bitwise_and > 0; //needs clarification why is it that the result of bitwise & is > 0 means its a dupe? if (already_bit_flagged) return false; using_int_as_32_flags |= using_0001_and_shifting_it_by_the_numeric_representation; } return true; } </code></pre>
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